Energy consumption in residential and commercial buildings is estimated to account for 21% and 18% respectively of primary energy consumption in the USA. Given the projected decline in the production of fossil fuels and the growth in global demand for energy it is recognized in both the public and private sectors that reducing energy consumption in residential and commercial buildings is an international imperative.
The commonest construction system used in the USA and in many other countries in both the residential and the commercial sectors is a structural frame that is covered by a weatherproof cladding. The structural frame is designed to withstand all the loadings on the building due to the weight of the building itself and of the finishes, occupants and equipment inside the building. Depending on the geographical location of the building it may also be designed to accommodate additional loadings due to external environmental factors such as snow, wind or seismic activity. The structural frame also provides the support for the windows, doors and other penetrations of the building envelope. The cladding material is designed to prevent the ingress of rain, moisture, air and other undesirable elements into the building and is commonly selected based on a combination of performance and aesthetic considerations.
Although different materials are used in residential and commercial construction the principles of this frame and cladding building system are the same in both sectors. Residential and light-commercial construction typically uses wood as the framing material while larger commercial construction uses steel. Residential and light-commercial construction commonly uses wood products as the exterior cladding (e.g. clapboards, shingles) although other materials (e.g. vinyl, fiber-cement) are increasingly used for cost and durability reasons. Commercial buildings use a variety of claddings including sheet metal, glass, cementitious boards, stucco and the like. Additional materials may also be used between the structural frame and the cladding to improve the resistance to penetration of moisture and air into the building.
For example, a very common wall system for residential and light commercial construction consists of the following sequence of materials from inside to out:    Gypsum-board or paneling. This has no structural function but provides a decorative interior finish and contributes to air sealing.    Wood framing. Commonly built of 2″×4″ or 2″×6″ wood lumber, the framing provides the vertical strength to support the weight of the building. Window and door penetrations are framed in lumber and structural headers are installed to transmit the loads around these penetrations. Floor, ceiling and roof systems normally rest on the top of exterior wall sections and load-bearing interior walls.    Sheathing. A sheathing material is mechanically fixed to the exterior of the wood framing. Commonly used materials are plywood and oriented-strand board (“OSB”). The sheathing may serve several functions including an air barrier, bracing the wall system against racking forces and providing a nailing surface for the exterior cladding. Note that in some applications sheathing may be omitted providing that a bracing system is installed to resist racking forces.    Housewrap. An air/water barrier is usually applied over the sheathing. Commonly a non-woven polyolefin material is stapled to the sheathing but historically an asphalt-impregnated paper based material (“builders felt”) was used for this purpose. The function of the air/water barrier is to reduce air infiltration into the building and to prevent moisture from cladding leaks (e.g. wind-driven rain) from entering the building.    Cladding. The cladding material is commonly installed by nailing the clapboard, shingle or other material to the sheathing. This practice also has the effect of creating numerous holes in the air/water barrier, thereby compromising its performance. It is a best practice, although rarely implemented, to leave a space between the cladding and the air/water barrier thereby creating a clear path for bulk water to drain down the face of the air/water barrier (creating a “drainage plane”) and for air to circulate between the cladding and air/water barrier to dry out any moisture that may have penetrated the cladding.
The same principles are commonly employed in the roof system in residential and light-commercial construction. From interior to exterior the sequence is:    Gypsum-board. This is usually only installed when the underside of the roof is part of the living or occupied area (e.g. cathedral ceilings). It is rarely applied in unfinished space such as attics    Wood framing. Since roof loads include a substantial lateral force component the dimensions are typically 2″×10″ or 2″×12″ to resist bending.    Sheathing. Similar sheathing materials are used as in the wall system    Air/water barrier. An asphalt-impregnated paper product (“builders felt” or “roofing felt”) is commonly used for this purpose. Synthetic or bituminous membrane products may be used in more vulnerable areas such as near the eaves or in valleys where roof sections join.    Cladding. The most common cladding material is asphalt-impregnated fiberglass reinforced shingles. Other materials include natural and synthetic slate, tiles and sheet metal.
Until the 1970's it was quite common for buildings to be built with no insulation. Since then rising energy prices and the introduction of energy codes have resulted in universal improvements in the insulation of buildings although current code levels still fall short of what many building scientists recommend.
The commonest way to insulate residential and light-commercial buildings of the frame-and-cladding type has been to install insulation in the framing cavities bounded by the gypsum-board on the inside, the sheathing on the outside and framing members on the sides and top and bottom. Fiberglass batt insulation continues to be the predominant cavity insulation used in new construction but other materials including blown-in fiberglass or cellulose and spray foam insulation either alone or in combination with a fiberglass batt have seen increasing use, providing both greater thermal conductive resistance and improved air sealing to reduce energy losses through air infiltration. To increase the level of cavity insulation there has also been a shift in wall construction from 2″×4″ framing to 2″×6″ framing to increase the depth of the cavity and therefore the resistance to heat loss through the insulated cavity. Retrofitting cavity insulation in existing frame-and-cladding buildings is difficult and costly, the commonest method being to drill holes in either the interior gypsum-board or exterior sheathing in order to blow fiberglass or cellulose insulation into the framing cavities.
Although cavity insulation reduces the flow of heat through a building assembly such as a wall or roof it suffers from a major weakness—heat flow through the framing members. Framing members may represent from 10% to well over 50% of the surface area of a wall depending on the number, area and location of windows, doors and corners. For energy calculations an average wall is typically assumed to have 20-25% framing. The thermal conductivity of wood framing is over three times greater than common insulation materials such as fiberglass batts or cellulose and the framing members therefore create thermal bridges that account for at least 30% of the heat loss through a wall or roof assembly.
Several strategies have been developed to overcome this thermal bridging including double wall construction, structural insulated panels and exterior insulation systems.
In double wall construction a second wall is framed inside the exterior structural wall with a gap of from 1″ to 6″ between the walls. Whenever possible, the framing members of the second wall are offset from the first wall to minimize thermal bridging through the framing. The gap between the two walls and the cavities of both walls are completely filled with an insulating material such as blown in fiberglass or cellulose or spray foams. This strategy is quite effective although expensive due to the material and labor cost of installing the interior framed wall.
The second strategy that has been used is to build the framing walls and/or roof sections of Structural-Insulated Panels (“SIPs”). SIPs consist of a sheet of insulating foam sandwiched between and adhered to two skins or sheets of plywood or OSB. The resulting panels are dimensionally stable and may be used as structural members either alone or in combination with large-section timbers that create a primary frame to which the SIPs are fastened (“timberframe construction”). Other than the insulating foam, there is no connection between the interior and exterior skins so SIPs must be installed so that the stresses due to the weight of the interior finish (e.g. gypsum board) or the exterior cladding are transmitted to the foundations directly through the skin panels. Although SIPs provide effective insulation and air-sealing they have found limited acceptance due to their cost, installation difficulties and the difficulties of installing electrical outlets and other services without compromising the structural and thermal integrity of the panels. These limitations may necessitate installing a non-structural wall against the interior side of the panel with significant additional material and labor cost or providing surface mounted raceways for electrical circuits that compromise the finished appearance of the wall.
The third strategy that has been used is to install an exterior insulation system to insulate the entire building. The commonest system of exterior insulation is to cover the exterior of the framing system with rigid expanded-foam boards and a less common approach, used principally in retrofit applications, is to build a framework on the outside of the framing wall to hold either a blown-in or a spray foam insulation system.
Rigid expanded foam boards consist of a sheet of expanded foam that is either unfaced or may have a protective sheet of plastic, foil or other material adhered to one or both faces. In one commercially available system the foam sheet is adhered to a composite wood-based material that may be applied directly to the wood structural frame to serve the bracing function of the sheathing material in conventional construction (“Structural Insulated Sheathing”). The expanded foam board in all these applications is factory-manufactured and may be made of a variety of low-thermal conductivity materials but polystyrene, polyurethane and polyisocyanurate foams are widely used. The foam boards are applied to the exterior of the sheathing in one or more layers and may be secured to the sheathing by mechanical (nails, screws) and/or adhesive systems. Joints between boards may be caulked and/or taped to provide an air-tight enclosure.
Although rigid expanded foam boards potentially provide high thermal insulation and air-sealing the system has significant limitations, principally related to the application of cladding material to the exterior of the foam board. In some applications the cladding material is installed by nailing through the foam board and into the structural sheathing and framing members behind. This approach however can only be used with foam board up to a limited thickness depending on the weight of the cladding material, and for this reason 2″ foam board is the maximum that vinyl siding manufacturers will allow while the manufacturers of fiber cement siding allow a maximum foam thickness of 1.5″. Other drawbacks to this approach are the large number of small thermal bridges where highly conductive steel nails penetrate through the insulation and the sheathing into the framing and the large number of nail holes in the air/water barrier and insulation board that potentially compromise performance.
If rigid expanded foam board thicker than that permitted by the cladding manufacturer is to be applied to a building then it is necessary to apply furring strips to the face of the foam board on which the cladding may be installed. Such furring strips are typically strips of wood or plywood that are aligned vertically with the framing studs in the structural wall and then long screws are driven through furring strip, the insulation and the sheathing into the framing studs. Although effective this method of using thick layers of insulating board covered by furring strips is rarely used because of the high cost of the long screws required and the amount of skilled labor time required to correctly install the furring strips.
Another approach to exterior insulation is to install a structure on the exterior wall that will support the cladding and to which an insulating system is then applied. This method is rarely used in new construction due to its cost and complexity but is sometimes used to significantly increase wall insulation levels in existing construction. In one application of this method a series of I-beams consisting of wood flanges joined by a web of plywood or OSB are attached vertically to the sheathing coincident with the internal framing studs by screwing through one of the flanges thereby creating a series of cavities on the exterior of the building. Insulating spray foam, commonly polyurethane based, is sprayed into the cavities, typically to less than the full depth of the I-beam. The cladding is then applied to the exterior flange of the I-beam creating a cavity between the cladding and the insulation through which air can circulate to dry out any moisture that penetrates the cladding. In another application of this method the I-beams are installed as described and then plywood or other sheet material is applied to the exterior flange face and the upper and lower ends of the I-beams to create a series of closed cavities. Holes are then drilled in the exterior cavity faces and fiber glass or cellulose insulation is blown in to insulate the cavity. Cladding is then applied to the exterior sheet material. Both methods can be used to create thick walls with very high insulating values but are both materials and labor intensive and therefore costly.
In U.S. Pat. No. 6,279,293 B1 Ojala discloses an insulated roof panel in which sheathing material on either side of an insulated panel is secured to a plurality of web trusses comprising top cords, bottom cords and a plurality of webs joining the cords together. This assembly is intended only for roofs and the web trusses are supplied to carry compressive forces. Ojala's panels result in a heavy construction and have a high level of thermal bridging through the web trusses.
In US Patent Application Publication 2006/0263575 A1 Ritchie discloses a method for reinforcing a foam panel using transverse strips of sheathing attached to sheets of material that comprise the two faces of the panel, the cavities between the two sheets being filled with foam. The reinforcement is intended to increase the rigidity of the panel and the intended use of the panels is roofing and flooring, not wall construction. Thermal bridging through the reinforcing strips remains a problem.
Accordingly there remains a need to provide an insulating structure which provides thermal insulation and resistance to air and moisture penetration, and can be made arbitrarily thick while still suitable for mounting of cladding materials without requiring thermal bridges that degrade thermal insulating capabilities.